5-16
Functional
Description
Portable
PLUS
Computer
When the logic input
FLOAT,
provided by the PPU,
is
set to logic
1,
the battery charger regulates at
float voltage. A logic
0 at
FLOAT
causes the battery charger to regulate at overcharge voltage.
The battery charger circuit has a low-current charge mode. In the event that the three-cell battery has
one or more shorted cells,
it
is
unsafe to charge the battery to the full float or overcharge voltage.
To
avoid over-charging such a battery, the low-current mode
is
enabled
and
the voltage regulation mode
disabled
when
VBAT
is
less than about Sv. Approximately
10
rnA of charge current
is
provided
when
the battery charger is in the low-current charge mode.
Two
outputs are provided by the battery charger to the PPU circuitry. POWER*
is
driven to logic 0 to
indicate that a recharger is plugged in
and
providing adaquate power to operate the battery charger.
POWER* becomes a high impedance to indicate that the battery charger is not operating. The HIGH
CURRENT* output is driven to logic 0 to indicate that the battery charger
is
providing a fairly high
load current, roughly
200
rnA
or more. HIGH CURRENT* becomes a high impedance to indicate that
the battery charger load current has dropped below
200 rnA.
The battery charger regulator is a linear design
and
operates in a similar fashion as the VCC regulator.
A2QSO
is the pass transistor that is turned
on
the proper amount to maintain
VBAT
at the regulated
voltage level. The input power is provided from the recharger, which produces 11.6
Vrms
when
open
circuited. A2CR64, A2CR6S, A2CR66, A2CR67,
and
A2CSO
form a full-wave rectifier which converts
the recharger ac output voltage to dc voltage, which
is
provided to the emitter of the pass transistor,
A2QSO.
A2USO,
a lead-acid battery charger integrated circuit, provides base current to control the pass transis-
tor. A resistor divider string made
up
of A2RS4,
A2RSS,
A2RS6, A2RS7,
and
A2CRS3 generate a
voltage that
is input at pin
13
and
compared with a reference voltage internal to
A2USO
(assume that
A2QSl is
turned
fully on). The internal reference voltage
is
2.3V ± 1 % at 2S°C.
A2USO
passes the
appropriate amount of base current for the pass transistor through pins 16
and
IS to maintain a
voltage at pin 13 equal to the internal reference voltage. When
FLOAT
is
driven to logic 1 by the PPU,
A2CRS3 is reversed-biased, no current flows through A2RS7,
and
the regulated voltage is determined
by
A2R54, A2RSS,
and
A2R56 only. This results in the battery charger operating in
'float'
mode:
7.0SV at
VBAT
when
the internal reference is
2.3V.
When
FLOAT
is
grounded, current flows through
A2RS7,
and
the battery charger regulates at the overcharge voltage.
The resistor divider string made
up
of A2RS4,
A2RSS,
A2RS6, A2RS7,
and
A2CR53 also provide volt-
age to pin 12 of
A2USO,
except at a different tap-off point. When the voltage at pin 12
is
above the
internal reference voltage, the voltage regulation mode is enabled; otherwise the battery charger oper-
ates in the low-current charge mode. In the low-current charge mode, charge current
is
sourced
by
pin
11
of
A2USO
through A2R53,
and
no
base current
is
provided for
A2QSO,
so
it
shuts off.
The
POWER* output
is
supplied by a voltage detector circuit in
A2USO.
When
an
input voltage of
4.8V or more
is
supplied to pin S of
A2USO,
the voltage detector circuit pulls the POWER* output
down
to logic 0
(A2USO,
pin
7).
When the input voltage supplied to
A2USO,
pin 5 is less than
4.2V,
POWER* becomes a high impedance. The PPU circuitry
is
able
to
detect whether POWER* is a logic
o or a high impedance,
and
hence whether the battery charger is charging the battery.
The HIGHCURRENT* output is provided by A2Q86. Base current for the main pass transistor of the
battery charger regulator flows through A2RS1. When there is little current through A2RSl, the volt-
age at the gate of A2Q86
is
too small to turn it on. HIGHCURRENT* remains a high impedance,
indicating to the
PPU that the battery charger regulator is supplying relatively little current. When the
regulator
is
providing a large amount of current, the base current through A2RSI
is
also large,
and
the
voltage at the gate of A2Q86
is
high enough to turn
it
on, pulling HIGHCURRENT* to logic
O.
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